Single chip microcontrollers have been known for over ten years. These devices are a complete computer on a chip including a central processing unit (CPU), random access memory (RAM) for data, an electrically programmable read-only memory (EPROM) for program memory, or alternatively read only memory (ROM) and various registers, timing circuits, latches, ports and interface circuits. One of the earliest microcontrollers made was the 8051, first manufactured by Intel Corporation in 1981. This part featured separate program and data memories with unique addresses for each. Previously, the 8051 has been a platform for other microcontroller improvements by others. See U.S. Pat. No. 4,782,439 to Borkar et al. relating to improved memory access and U.S. Pat. No. 4,780,814 to Hayek relating to a communications interface.
Smart cards contain single chip microcontrollers which combine a CPU with various types of memory including RAM, ROM, EPROM and EEPROM in combination with various ports, memory access and even a gate array for logic. Such cards use EPROM memory for program storage and all other memory for data storage.
One of the problems with EPROM storage is that UV-light exposure is required to erase the device prior to reprogramming. Where reprogramming is required in the field, it is frequently easier to replace the entire controller with a fresh one than to attempt an erase and rewrite operation for the program memory. EEPROMs have not been considered as program memories because array size has been thought to be incompatible with microcontroller functions, except perhaps for very small arrays.
While microcontrollers operating on low voltage power supplies exist in the prior art, such supplies are not suitable for EEPROMs which require voltages above 17 V for programming.
An object of the invention was to devise a field re-programmable microcontroller having a self-contained program memory.
Another object of the invention was to devise such a microcontroller operating on a single low voltage power supply.